Low-frequency radio navigation systems have evolved from the early Loran (LOng RAnge Navigation) system operating with pulsed frequencies between 1800 and 2000 kHz and the Loran-C/D systems operating at a frequency of 100 kHz. Each Loran station operates in a pulsed sequential mode such that only one out of three stations can be received at a time. During the transmission pulse period of the master station, both slave stations remain silent, after which the slave stations will transmit pulsed energy in sequence. The time differences between pulses received from the slaves, as compared to the master, provide hyperbolic navigation in an acceptable manner for most purposes, particularly over a long range of about 1000 km. For navigation over longer ranges, the Omega system operating in the 10.2 to 13.6 kHz band may be used, but accuracy is limited to 1 or 2 nautical miles at the present state of the art in predicting the phases of the Omega skywaves as functions of position and time. Omega transmits keyed (interrupted) CW signals from eight stations on several discrete frequencies utilizing multiplexing techniques.
For navigation over very short ranges, such as to 100 km from the transmitter, the pulsed format of Loran C/D is not necessary to discriminate the highly predictable groundwave from the unpredictable skywave. Therefore, continuous wave (CW) signals, keyed in an Omega-like format, can be used.
Instantaneous position and velocity may be determined by phase comparison of CW transmissions from three or more synchronized transmitters. Two CW systems offering such capability are presently in operation. One is the keyed CW Omega system referred to above, and the other is an unkeyed CW system known as Decca, which operates in the 70 and 130 kHz frequency band. A Decca receiver measures and integrates the relative phase difference between the signals from the transmitters which operate at different frequencies for identification. Tracor, Inc. of Austin, Tex., has studied the use of a keyed CW system for river and harbor navigation, and Collins Telecommunications Systems Division of Rockwell International has amplified on that study.
The present invention implements a keyed CW system to provide radio navigation for the Army's proposed Mobile Automated Field Instrumentation System (MAFIS) using the highly predictable groundwave from a plurality of transmitters which transmits a single frequency or multiple frequencies in a time multiplexed pattern. The choice of frequency is not critical, but frequencies in the range 200-500 kHz offer the widest groundwave coverage by minimizing skywave effects as a function of range. MAFIS is to be used for command and control, communications, weapons-engagement, and navigation (position and velocity) of up to 2000 receivers, where each receiver may be assigned to a soldier, tank or other mobile unit. This application requires 15-meter accuracy over areas as large as 50.times.50 km. Such a requirement cannot be met by any existing operational system.